Studies using narrow-band spectral lights reveal three rod influences on hue perception: a red bias at short wavelengths, a blue bias at middle wavelengths, and a green bias at long wavelengths). Studies using less saturated lights have shown only some of these effects. Here, we assessed rod influence on mixtures of pairs of narrow-band lights that spanned desaturated regions of color space. The results reveal rod influence on R/G and B/Y opponent-hue dimensions at chromaticities extending from the spectrum locus toward white.

Consistent with prior studies, along the spectrum locus, rods shifted all three unique hues to longer wavelengths: by up to 50 nm for unique green (blue bias), 10 nm for unique blue (red bias), and 9 nm for unique yellow (green bias). All three rod hue biases were also found for desaturated lights, extending toward white for distances that varied among observers. Some observers showed rod effects for stimuli of as little as ∼25% excitation purity, the least saturated we have tested so far. Rod influence was generally more pronounced for desaturated blues and greens than for desaturated yellows, even when light levels were adjusted to roughly equalize rod excitation for the three color ranges.

These results show that all three rod hue biases can alter the appearance of a wide range of desaturated hues like those found in the natural world, not just spectral hues of the laboratory.